Most modern transonic transport aircraft such as Lockheed Corporation's L-1011 transport cruise at Mach numbers between approximately 0.80 and 0.87. Further increases in speed tend to cause strong shock waves on or about the airfoil (wing) which cause a marked increase in drag. At lower Mach numbers the value of the coefficient of drag is comprised mostly of induced and skin friction phenomena. The coefficient of drag increases dramatically with increasing Mach number due to wave drag and peaks at a Mach number of one or thereabouts. Thus, modern aircraft flying in this high subsonic or transonic range have airfoils designed to delay the onslaught of this wave drag above a cruise Mach number determined from initial and operating cost considerations.
This has classically been accomplished by designing the airfoil with a sophisticated curvature. For example, U.S. Pat. No. 3,952,971 "Air Foil Shaped For Flight at Subsonic Speeds," by R. T. Whitcomb, uses an upper surface contoured to control flow accelerations and pressure distributions over the upper surface to prevent or mitigate shock formation on the airfoil upper surface at flight speeds well above the critical Mach number. A more highly cambered than usual trailing edge section is provided which improves overall airfoil lifting efficiency. Unfortunately, the shape of this particular wing presents structural problems, particularly in relationship to designs for the incorporation of trailing edge flaps. Thus, the airfoils for this type of aircraft have been basically a compromise between aerodynamic efficiency and structural integrity.
It has also been recognized that wing trailing edge flaps may be deflected small amounts at cruise to improve spanwise loading and reduce drag. Of course, variations in wing camber can optimize performance over a wide range of flight conditions.
Furthermore, there have been successful attempts to increase the lift and reduce the drag of airfoils used on racing cars by incorporation of a trailing edge flap. On racing cars an inverted airfoil is often used to create downforce to better hold the car on the ground. It was discovered that a small flap projecting vertically upward normal to the chord at the trailing edge of the airfoil would increase down force by significant amounts with some reduction in drag. This automobile flap is discussed in detail in the AIAA Journal Of Aircraft, Paper No. 80-3034, entitled "Design of Air-Foils for High Lift" by Robert H. Liebeck.
What has not been hitherto exploited is the possibility of simultaneously altering chordwise loading, so as to alleviate high pressure peaks at the leading edge, and improve airfoil thickness distribution by use of an extremely small wedge flap which can easily be added to an existing wing.
Thus, it is a primary object of the subject invention to provide an airfoil design for an aircraft that substantially increases the coefficient of lift and reduces the coefficient of drag at cruise, thus providing an overall increase in fuel efficiency.
It is another object of the subject invention to provide a simple device for improving the performance of an airfoil without undue detrimental changes to the overall performance of the aircraft.
A still further object of the subject invention is to provide a device for improving performance of an airfoil for an aircraft which can easily be attached without undue structural changes.